Light emitting diode (LED) is an opto-electronic element that has been widely adopted in the field of light source. Comparing with traditional light bulb or fluorescent Lamp, LED shows significant improvement in power saving and lifetime. Therefore, LED devices gradually replace traditional light source in many application such as traffic sign, backlight module, lamp post or medical equipment.
FIG. 1A is a diagram illustrating a conventional light emitting diode structure. As shown in FIG. 1, a conventional LED includes a substrate 10, a semiconductor stack layer 12 disposed on the substrate 10 and an electrode 14 disposed on the semiconductor stack layer 12, wherein the semiconductor stack layer 12, from top to bottom, includes at least a p type semiconductor layer 120, an active layer 122 and a n type semiconductor layer 124. In addition, in the conventional LED 10, the active layer 122 is a multiple quantum well (MOW) structure. It means the active layer 122 is composed of a plurality of quantum well layers 126 and a plurality of barrier layers 128 which stack with each other.
The light emitting mechanism of the LED 100 is, by injection of electrons and holes respectively from the n type semiconductor layer 124 and the p type semiconductor layer into the active layer 122, to recombine electrons e and holes h in the quantum well layers 126 of the active layer 122 and release energy by emitting photons thereafter. FIG. 1B is a diagram illustrating bandgap and light emitting mechanism in a conventional LED. As shown in FIG. 1B, since the carrier mobility of holes h is lower than the carrier mobility of electrons e, most of the recombination of electrons e and holes h in LED 100 happens in a portion of the quantum well layers 126 near the p type semiconductor layer 120. Therefore, the light emitting region in active layer 122 concentrates in only a few of the quantum well layers 126 close to the p type semiconductor layer 120, so as to only a few of the structure in the active layer 122 emits light.